1. Field of the Invention
This invention relates to a wave-processing method and a wave-processing die for a core metal of a wet friction material, which gains torque by applying high pressure to an opposite face while dipped in an oil and which is made by joining a friction material substrate to a ring core metal by adhesion.
2. Description of the Related Art
A technique has been developed for waving or undulating a core metal plate of a wet friction material so as to absorb shock in engaging a friction material with an opposite face of a pressure plate. This enables the clutch to smoothly engage. With such technique, the ring-shaped core metal is given a wave shape or undulation in its circumferential direction. Thus, a wave or undulation is provided on a face of a wet friction material that is stuck to the core metal. Specifically, there are two main methods of giving the wave to the core metal of the wet friction material. These conventional methods are described referring to FIG. 14 and FIG. 15. FIG. 14 is a schematic view showing a conventional cool working process. FIG. 15 is a schematic view showing a conventional hot working process.
The cool work process is described referring to FIG. 14. As shown in FIG. 14, in the cool working process, a core metal 53 made by stamping is kept pressed between a punch 51 and a counter punch 52 of a die 50 having a wave shape. Thus, the wave shape of the die 50 is transferred or imparted to the core metal 53. This method is relatively simple and has good productivity. However, a wave height given to the die 50 cannot be imparted to the core metal as it is, due to “return” phenomenon after releasing pressure. Therefore, in general, it is necessary to form the wave shape of the die 50 at a height several times to several dozens times as large as a required wave height. Moreover, the “return” of a core metal steel must be taken into account.
On the other hand, in the hot working process, as shown in FIG. 15, a plurality of core metals 58 are stacked in layers. Then, the core metals 58 are heated at a high temperature of 400 to 500 degrees centigrade while pressed between an upper mold 56 and a lower mold 57 of a die 55 having a wave shape. Thus, it restrains the “return” of the core metal steel. With the hot working process, the wave shape 58a can be obtained stably, since the wave shape is imparted to the core metal 58 while relieving and removing internal stress of the core metal 58 by heating.
In the cool working process shown in FIG. 14, there take place very different “return” phenomenon on the core metal steel depending on various factors. Particularly, it depends very much on a history of the steel material. Specifically, it is difficult to satisfy a stable wave height even if the core metal is pressed by the same wave-processing die 50, depending on factors such as a steel material lot, rolling history, rolling direction, etc. Thus, there is much variation in the waves 53a formed by the cool working process. Consequently, the cool working process cannot satisfy wave accuracy required for the wet friction material.
On the other hand, in the hot working process show in FIG. 15, high wave accuracy can be stably obtained. Therefore, the hot working process is adopted as the waving process these days. However, the hot working process has complicated steps as compared with the cool working process. Moreover, the hot working process needs longer processing time and is inferior in productivity. Furthermore, with the hot working process, the core metal must be processed at a high temperature. Then, it has disadvantages that huge amount of energy is consumed and that production costs increase inherently.